I know that you can easily determine the percent capacity remaining in a battery using an arduino by dividing the voltage down to something that the ADC can read. What I'm stuck on is how the voltage will vary as the battery discharges. I've looked at multiple datasheets and tried to look on Duracell's website for one (I'm using a Duracell Ultra SLA battery), but I couldn't figure out how the voltage behaves as the battery discharges.
The main thing I need to figure out is the voltage at which the battery is considered "dead." But I think I also need to figure out how to read the voltage curves in the datasheets because (from what I can tell) the voltage does not vary linearly with the battery's capacity remaining. Can anyone point me in the right direction for information about this?